Promsafe introduced: Type Safe Labels. (Draft, Discussion is still re…

…quired)

prometheus/promsafe/labels_provider.go

@@ -0,0 +1,161 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package promsafe
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// LabelsProvider is an interface that allows to convert anything into prometheus.Labels
+// It allows to provide your own FAST implementation of Struct->prometheus.Labels conversion
+// without using reflection.
+type LabelsProvider interface {
+	ToPrometheusLabels() prometheus.Labels
+	ToLabelNames() []string
+}
+
+// LabelsProviderMarker is a marker interface for enforcing type-safety of StructLabelProvider.
+type LabelsProviderMarker interface {
+	labelsProviderMarker()
+}
+
+// StructLabelProvider should be embedded in any struct that serves as a label provider.
+type StructLabelProvider struct{}
+
+var _ LabelsProviderMarker = (*StructLabelProvider)(nil)
+
+func (s StructLabelProvider) labelsProviderMarker() {
+	panic("LabelsProviderMarker interface method should never be called")
+}
+
+// NewEmptyLabels creates a new empty labels instance of type T
+// It's a bit tricky as we want to support both structs and pointers to structs
+// e.g. &MyLabels{StructLabelProvider} or MyLabels{StructLabelProvider}
+func NewEmptyLabels[T LabelsProviderMarker]() T {
+	var emptyLabels T
+
+	val := reflect.ValueOf(&emptyLabels).Elem()
+	if val.Kind() == reflect.Ptr {
+		ptrType := val.Type().Elem()
+		newValue := reflect.New(ptrType).Interface().(T)
+		return newValue
+	}
+
+	return emptyLabels
+}
+
+//
+// Helpers
+//
+
+// promsafeTag is the tag name used for promsafe labels inside structs.
+// The tag is optional, as if not present, field is used with snake_cased FieldName.
+// It's useful to use a tag when you want to override the default naming or exclude a field from the metric.
+var promsafeTag = "promsafe"
+
+// SetPromsafeTag sets the tag name used for promsafe labels inside structs.
+func SetPromsafeTag(tag string) {
+	promsafeTag = tag
+}
+
+// iterateStructFields iterates over struct fields, calling the given function for each field.
+func iterateStructFields(structValue any, fn func(labelName string, fieldValue reflect.Value)) {
+	val := reflect.Indirect(reflect.ValueOf(structValue))
+	typ := val.Type()
+
+	for i := 0; i < typ.NumField(); i++ {
+		field := typ.Field(i)
+		if field.Anonymous {
+			continue
+		}
+
+		// Handle tag logic centrally
+		var labelName string
+		if ourTag := field.Tag.Get(promsafeTag); ourTag == "-" {
+			continue // Skip field
+		} else if ourTag != "" {
+			labelName = ourTag
+		} else {
+			labelName = toSnakeCase(field.Name)
+		}
+
+		fn(labelName, val.Field(i))
+	}
+}
+
+// extractLabelsWithValues extracts labels names+values from a given LabelsProviderMarker (parent instance of a StructLabelProvider)
+func extractLabelsWithValues(labelProvider LabelsProviderMarker) prometheus.Labels {
+	if any(labelProvider) == nil {
+		return nil
+	}
+
+	if clp, ok := labelProvider.(LabelsProvider); ok {
+		return clp.ToPrometheusLabels()
+	}
+
+	// extracting labels from a struct
+	labels := prometheus.Labels{}
+	iterateStructFields(labelProvider, func(labelName string, fieldValue reflect.Value) {
+		labels[labelName] = stringifyLabelValue(fieldValue)
+	})
+	return labels
+}
+
+// extractLabelNames extracts labels names from a given LabelsProviderMarker (parent instance of aStructLabelProvider)
+func extractLabelNames(labelProvider LabelsProviderMarker) []string {
+	if any(labelProvider) == nil {
+		return nil
+	}
+
+	// If custom implementation is done, just do it
+	if lp, ok := labelProvider.(LabelsProvider); ok {
+		return lp.ToLabelNames()
+	}
+
+	// Fallback to slow implementation via reflect
+	// Important! We return label names in order of fields in the struct
+	labelNames := make([]string, 0)
+	iterateStructFields(labelProvider, func(labelName string, fieldValue reflect.Value) {
+		labelNames = append(labelNames, labelName)
+	})
+
+	return labelNames
+}
+
+// stringifyLabelValue makes up a valid string value from a given field's value
+// It's used ONLY in fallback reflect mode
+// Field value might be a pointer, that's why we do reflect.Indirect()
+// Note: in future we can handle default values here as well
+func stringifyLabelValue(v reflect.Value) string {
+	// TODO: we probably want to handle custom type processing here
+	//       e.g. sometimes booleans need to be "on"/"off" instead of "true"/"false"
+	return fmt.Sprintf("%v", reflect.Indirect(v).Interface())
+}
+
+// Convert struct field names to snake_case for Prometheus label compliance.
+func toSnakeCase(s string) string {
+	s = strings.TrimSpace(s)
+	var result []rune
+	for i, r := range s {
+		if i > 0 && r >= 'A' && r <= 'Z' {
+			result = append(result, '_')
+		}
+		result = append(result, r)
+	}
+	return strings.ToLower(string(result))
+}